I have a vacation home with a septic smell that only manifests itself when the house is closed and the gas furnaces or HW heater are running. I have never arrived at the home and smelled the odor; it takes a day or two(could be a few hours) to produce the smell.. This is despite the fact that in the winter we keep the furnace at 50 F (temp is at 10-20 F outside) I should also note that it happens on cool fall days, so I dont think it is a frozen vent .

All of the Studor vents have been replaced. I have also had the system pressure tested and had two smoke bombs set off in each of the two vent stacks.